Advice needed on searching for an air leak

I'm looking for advice on searching for an air leak.
The truck is an 06 Freightliner Coulumbia with a Mercedes MBE 4000.
Usually the truck leaks down to around 50lbs in about an hour when parked with the engine off. The leak does not occur all of the time. It has now been parked for over 4 hours with no loss of pressure. The fastest it has ever leaked was 25lbs every three minutes while driving. But it stopped after about 30 minutes and didn't leak anymore. I have not been able to hear any air leaking out.
The only time that it does not leak is when it is idling with the engine fan turned on. (Makes me think it is the engine fan air clutch)
It does not leak fast enough doing a pretrip brake inspection and has been through 2 DOT scale house inspections in the last 10 days.
This leak is starting to drive me crazy. Anyone have any ideas?

I had a similar situation a couple of years ago and I got the traditional suggestions of using smoke or soapy water to find the leak. Smoke you have have a machine for, and not all shops have them. Before I went that route, the leak would have to be bigger. Soapy water is not practical if your running down the road, and I wasn't comfortable putting soap in the air system. I thought about Fluorescent A/C Leak Detection Dye, but I figured that the air drier would get rid of it before it had a chance to circulate through the system. I had to figure a way to slow it down enough to circulate.

The solution I came up with was PAG oil. It's also used in A/C systems. It's a very light, clean, non-acidic oil that keeps a/c O rings lubricated. I mixed 3 ounces of PAG oil with two bottles of dye and put the mix in at the compressor. It took about two days for the drier to purge the mixture and another day to find the leak. I didn't even need the black light or glasses to see the dye as there was a thin streak of oil down the side of one of the airbags on the cargo box of the truck. It was from a pinhole leak at the top of the airbag. When the bag was full with no cargo, it would leak. If I was loaded, the bag would compress and the upper base of the bag would seal the leak. If the engine was off, the bag would leak until the base came down on the hole and sealed it.

I'm not saying this is a foolproof method, but it worked for me. You might end up waiting until the leak gets big enough to hear. If you do use this, make sure you keep cleaning your purge valve on your drier until the oil is purged. It holds onto grime like all oil does, and you don't want any excess grime to get into your O rings on the valve.

Could very well be a leak at the fan or the air hose to it. Not uncommon at all. As mentioned, check the purge valve at the dryer because it could be sticking at times causing a intermittent leak. Freightliner is notorious for replacing dryer assemblies whether it needs it or not because of that valve.
We had several installed only to find out the leak was at the compressor. That is the difference between an actual technician and a parts changer.

The load leveling valve on your rear end could be your problem. They can do crazy things when truck is sitting in different positions. Just because truck is on level ground does not mean it won't discharge a little air. If you find that when truck is in a certain position it leaks faster, then I would suspect the leveling valve. Replacing it doesn't mean it will stop it either. I have had 2 different trucks that would drain air while sitting from the valve. It is not affecting the brakes and does not overwork the compressor, so after knowing what it was, I just live with it.

As for finding a leak, I use soapy water in a spray bottle. If I don't find the leak right away, I go back after it has set awhile and see if I see a big mound of small bubbles anywhere. A tiny leak is hard to detect unless you let the soapy solution set for a minute or so.

I am beginning to think I am losing my mind.
I am at home OOS and my number one mission was to find and fix this leak. It has been worrying me for two weeks.
The truck sat last night for 15 hours and did not leak.
This morning I pumped the brakes down, started the truck and moved it. 2 hours and still no leak.
The only thing I did is have the truck washed yesterday on the way home.
Could the pressure washers have blown a piece of dirt or some foreign object out of a pressure relief valve or somehow caused it to stop leaking?
I don't want to go back on the road until I am sure it is fixed.

It finally started leaking again. (It feels strange being happy about a leak.)
It is coming from the Meritor Wabco ABS modulator valve.
After running the truck to refill the tanks I could hear a howling sound coming from under the truck near the rear axle. When I crawled under I could hear and feel the air coming out. Lost 75lbs in 5 minutes.
So now I know what I need to have repaired.
Anyone care to venture a guess on how much it will cost?

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